The impact modifiers have been used heretofore for endowing thermoplastic resins with impact resistance. As the impact modifier, there is widely known, for example, a graft copolymer which is prepared by graft-polymerizing a vinyl monomer to a butadiene rubber having a low glass transition temperature (hereinafter referred to as "Tg"). However, the thermoplastic resin having excellent thermal stability and weather resistance has not been obtained because such a graft copolymer is thermally unstable due to its unsaturated bonds.
Also as the impact modifier, there is known a graft copolymer prepared by graft-polymerizing a vinyl monomer to an acrylic rubber. Such a graft copolymer exhibits excellent improved effects in thermal stability and weather resistance, but since it has a relatively high Tg, significant effect in improvement of impact resistance cannot be obtained. Also since the acrylic rubber used as a material for the graft copolymer has a low refractive index, there is a defect that transparency of a molded article of the thermoplastic resin to which the graft copolymer has been added is impaired.
JP-A-60-252613 and JP-A-2-8209 disclose, as an impact modifier having a low Tg and being excellent in thermal stability and weather resistance, a graft copolymer prepared by graft-polymerizing a vinyl monomer to a polyorganosiloxane (silicone rubber). Though improved effect in impact resistance can be obtained to a certain extent, it is still insufficient and much higher improvement is required. Also there is a defect that a molded article of the thermoplastic resin to which such a graft copolymer has been added has poor surface gloss. Further since the polyorganosiloxane has a low refractive index, there is a defect that transparency of the thermoplastic resin molded article is impaired.
Further JP-A-64-6012 and JP-A-4-100812 disclose, as an impact modifier having excellent thermal stability and weather resistance without lowering surface gloss of a thermoplastic resin molded article, a graft copolymer prepared by graft-polymerizing a vinyl monomer to a composite rubber comprising a polyorganosiloxane (silicone rubber) component and poly(alkyl (meth)acrylate) (acrylic rubber) component. However, since the polyorganosiloxane has a low refractive index, there is also a defect that transparency of the thermoplastic resin molded article is impaired by adding the impact modifier containing such a silicone rubber.